Non-standard
This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.

Obsolete since Gecko 60 (Firefox 60 / Thunderbird 60 / SeaMonkey 2.57)This feature is obsolete. Although it may still work in some browsers, its use is discouraged since it could be removed at any time. Try to avoid using it.

Non-standard. Do not use!
The expression closure syntax is a deprecated Firefox-specific feature and has been removed starting with Firefox 60. For future-facing usages, consider using arrow functions.